Cold seal release film with improved scuff resistance

ABSTRACT

A cold seal release film in the form of a monolayer film or an outer layer of a multilayer film. The cold seal release film includes a nucleating agent and 10%-100% by weight of the polymer composition of a polymer selected from the group consisting of 1-butene homopolymer, 1-butene/ethylene random copolymers having over 50% by weight 1-butene therein blends of the 1-butene homopolymer and the 1-butene/ethylene random copolymers, and 90%-0% by weight of the polymer composition of a polymer selected from the group consisting of ethylene random copolymers of polypropylene, butene random copolymers of polypropylene and ethylene-butene random terpolymers of polypropylene.

RELATED APPLICATIONS

This application is a continuation of pending U.S. application Ser. No.10/845,828 filed on May 14, 2004, titled COLD SEAL RELEASE FILM WITHIMPROVED SCUFF RESISTANCE, which, in turn, is a divisional of U.S.application Ser. No. 10/141,258 filed on May 8, 2002, and titled COLDSEAL RELEASE FILM WITH IMPROVED SCUFF RESISTANCE, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to cold seal release films, and moreparticularly to packaging structures employing. Cold seal release filmshaving improved scuff resistance.

Cold seal release films are most commonly used as the outside web orfilm of a multi-web packaging structure. In common practice, the releasefilm is a clear web that is reverse printed and adhesive laminated to aninner web such as an opaque film (in a candy bar wrap) or a metallizedfilm (in a breakfast bar wrap). After the two films are laminated alayer of cold seal adhesive is applied to the inner web as the web iswound into a roll. The cold seal release film serves the followingfunctions: 1) it provides a transparent web which can be reverse printedso that the decoration is “buried” beneath the clear web to prevent theink from being worn away; 2) it provides the desired slip properties sothat the laminate will run effectively on packaging machines and 3) itprovides a surface with low adhesion to the cold seal adhesive so thatthe laminate roll can be unwound during a packaging operation, with theadhesive remaining on the desired surface of the lamination.

The most successful prior art cold seal release film in the market placetoday employs a polypropylene homopolymer as the cold seal releasesurface. This prior art film provides good printing characteristics, lowcoefficient of friction (COF) and a reasonably low adhesive releaseforce from the most common cold seal adhesives; namely, acrylic/rubberlatex adhesives. Moreover, prior art cold release films employing apolypropylene homopolymer also have reasonably good scuff resistence.However, the prior art films employing polypropylene homopolymer have amajor deficiency; namely, they do not provide the desired low adhesiverelease force from synthetic adhesives, e.g., ethylene vinyl acetatebased adhesives.

Other polymer films containing 1-butene homopolymer, 1-butene/ethylenecopolymers (collectively referred to as polybutenes) and 1-butenecopolymers blended with ethylene and/or butene random polypropylenecopolymers yield low adhesive release force from both syntheticadhesives and acrylic/rubber latex adhesives. However, these prior artfilms do not have desired scuff resistance properties. Representativepatents disclosing these latter polymer compositions for cold releasefilms are U.S. Pat. Nos. 5,981,047; 5,798,174; 5,489,473 and 5,792,549,which are all incorporated herein by reference.

As noted above, an important characteristic for cold seal release filmsis good scuff resistence. This is an important property because the filmserves as the outside web or layer in a monolayer or multilayer coldseal packaging application. Therefore, scuffing, which can occur eitherduring the printing/laminating process or the packaging process, causesthe finished product to have a dull or shop-worn appearance, which isundesirable.

The following additional patents also disclose films employing butenepolymers:

U.S. Pat. No. 4,554,321 to Hwo et al. discloses heat sealable filmshaving improved seal impact strength comprising a stereoregular,elastomeric polymer of butene-1 or blends thereof with conventionalbutene-1-ethylene copolymers, optionally with small amounts ofpolypropylene and/or crystallization nucleating agents. The use of suchstereoregular, elastomeric polymers is not desirable for use in coldseal release films of this invention.

U.S. Pat. No. 5,998,039 to Tanizaki, et al. discloses a polypropylenecomposite film comprising (1) a crystalline polypropylene layer and (2)a layer formed from a blend of a polypropylene homopolymer (5-95%) and apropylene 1-butene random copolymer (95-5%), wherein the propylenecomponent is greater than 50%, by weight, of the copolymer composition.

WO 00/32395 A1 to Chang et al. discloses a polypropylene film containinga non-migratory slip and release additive package of a synthetic ormineral wax, an aluminosilicate additive, and optionally silicone oiland a cross-linked silicone polymer resin to improve cold seal adhesiverelease properties. Although this patent discloses adding nucleatingagents to a polypropylene film for improving the strength of the film,there is no disclosure or suggestion that nucleating agents should orcould be added to a polymer composition including a poly-1 butenehomopolymer or copolymer composition.

Nucleating agents are known to increase the stiffness of processedmaterials. Nucleating agents also are known to improve the optical andbarrier properties of the resulting materials. Sodium benzoate is adesirable nucleating agent because, for among other reasons, it isreadily available and economically practical to use.

However, the use of nucleating agents such as sodium benzoate in theproduction of biaxially oriented polypropylene films has been consideredto be undesirable because, in the amounts typically employed, it createsprocessing difficulties. U.S. Pat. No. 5,837,177 to DiNardo et al.discloses a method of making a polypropylene product for use in makingbiaxially oriented films. The method includes the addition of animproved nucleating system that allows for even distribution of anucleating agent in a polypropylene material. The improved nucleatingsystem comprises a nucleating agent such as sodium benzoate and a liquidmiscible with the nucleating agent. The solution is combined with thepolypropylene material, and the miscible liquid is removed duringprocessing.

There is a need in the packaging field for an improved cold seal releasefilm exhibiting both a low adhesion to cold seal adhesives, includingsynthetic adhesives, and improved scuff resistance. It is to such filmsthat the present invention relates.

All references cited herein are incorporated herein by reference intheir entireties.

SUMMARY OF THE INVENTION

A cold seal release film constituting either a monolayer film or anouter layer of a multilayer film includes a nucleating agent, such assodium benzoate, provided in a polymer composition comprising 10%-100%by weight of the polymer composition of a polymer selected from thegroup consisting of 1-butene homopolymer, ethylene random copolymers of1-butene including over 50% by weight 1-butene and blends of the1-butene homopolymer and the ethylene random copolymers of 1-butene, and90%-0% by weight of the polymer composition of a polymer selected fromthe group consisting of random copolymers of ethylene and propylene,random copolymers of butene and propylene and random terpolymers ofethylene-butene-propylene, said propylene being over 50% by weight ofthe polymer composition in said random copolymers of ethylene andpropylene and random copolymers of butene and propylene and being themajor component by weight of the random terpolymers ofethylene-butene-propylene.

A cold seal release film in accordance with this invention includes anucleating agent, preferably sodium benzoate, added to a polymercomposition comprising 10%-100% by weight of the polymer composition ofa polymer selected from the group consisting of 1-butene homopolymer,1-butene/ethylene random copolymers having over 50% by weight 1-buteneand blends of the 1-butene homopolymer and ethylene random copolymers of1-butene, and 90%-0% by weight of the polymer composition of a polymerselected from the group consisting of random copolymers of ethylene andpropylene, random copolymers of butene and propylene and randomterpolymers of ethylene-butene-propylene. In the aforementioned film thepropylene is over 50% by weight of the polymer composition of the randomcopolymers of ethylene and propylene and the random copolymers of buteneand propylene, and is the major component by weight of the randomterpolymer of ethylene-butene-propylene. These films have beendetermined to have excellent cold seal release from a variety ofadhesives, including acrylic/rubber latex adhesives and syntheticadhesives, as well as excellent scuff resistant properties. Mostpreferably, the cold seal release from the synthetic adhesives is lessthan 50 grams/inch.

In accordance with this invention the 1-butene/ethylene randomcopolymers include over 50% by weight of the 1-butene monomer. Mostpreferably the 1-butene/ethylene random copolymer includes less than 10%by weight of ethylene and more preferably between 0.1 % and 6% by weightof ethylene. Representative 1-butene/ethylene random copolymers usablein this invention are Basell DP8220 and Basell PB8340, which are mediumethylene content and low ethylene content 1-butene random copolymers,respectively. In particular, Basell DP8220 includes approximately 2.5%by weight ethylene and Basell PB 8340 includes approximately 0.2% byweight ethylene. It is believed, based upon experience, that the1-butene homopolymer will perform in this invention the same as thebutene random copolymer including 0.2% by weight ethylene. Both of theabove-identified 1-butene/ethylene random copolymers are manufactured byBasell Polyolefins of Wilmington Del. 19808.

As noted above, the 1-butene homopolymer and 1-butene/ethylene randomcopolymers can be employed alone, can be blended together, or each canbe employed in combination with other copolymers and terpolymers,including ethylene/propylene random copolymers, propylene/butene randomcopolymers and propylene/ethylene/butene random terpolymers. Thepreferred ethylene/propylene random copolymers are predominatelypropylene monomers, by weight, including up to about 10% by weightethylene. More preferably the propylene/ethylene random copolymersinclude less than 6% ethylene, by weight. A representativepropylene/ethylene random copolymer usable in this invention is Fina8573, which includes approximately 5% by weight ethylene monomer in it.Fina 8573 is manufactured by ATO FINA Petrochemicals, LaPorte Texas,77571.

As noted above, another copolymer that can be employed in combinationwith either 1-butene homopolymer or a 1-butene/ethylene copolymer is apropylene/butene random copolymer, wherein the propylene is the majorcomponent, by weight, of the copolymer. Most preferably thepropylene/butene random copolymer includes less than 35% by weightbutene and more preferably 3% to 20% by weight butene. A representativepropylene/butene random copolymer usable in this invention is UnionCarbide DS4DO5, manufactured by DOW Plastics in Houston Texas 77042.This propylene/butene random copolymer includes approximately 14% byweight butene in it.

A representative propylene/ethylene/butene random terpolymer usable inthis invention is Sumitomo SP88. This terpolymer includes approximately85% by weight polypropylene, based on the weight of the polymercomposition of the terpolymer, approximately 12% by weight ethylenebased on the weight of the polymer composition of the terpolymer andapproximately 3% by weight butene based on the weight of the polymercomposition of the terpolymer. This terpolymer is manufactured byPhillips Sumika, Houston Texas 77058.

The preferred nucleating agent employed in this invention is sodiumbenzoate; preferably in the range of 0.02-0.1% of the overall polymerblend. It is considered to be within the scope of this invention toemploy other nucleating agents. Individuals skilled in the art can testvarious nucleating agents to determine whether they provide enhancedscuff resistance without adversely affecting the cold seal releaseproperties from the adhesive.

In the context of this invention a nucleating agent is one that has theability to raise the crystallization temperature of a polyolefin asmeasured by Differential Scanning Calorimetry (DSC). The quantity of thenucleating agent required to raise the crystallization temperature willvary depending upon the specific nucleating agent that is employed.

In accordance with this invention the cold seal release film can be amonolayer web or film, or an outer layer of a multilayer, coextrudedfilm. In common packaging applications the monolayer or multilayer,coextruded release films are adhered to yet another film, which also canbe a monolayer or multilayer film having additional desired propertiesfor specific packaging applications and also receiving a cold sealadhesive on the outer exposed surface thereof. For example, the otherfilm can be a single layer or multilayer white opaque film or ametallized film layer. In these latter applications the surface of themonolayer or multilayer, coextruded release film opposite the releasesurface, which is the surface to be laminated to the other film, istreated, preferably oxidatively treated by corona or flame treatment, tomake it receptive to laminating adhesives and, optionally, to printinginks. Thus, this latter surface optionally can be reverse printed so asto be viewable from the outer side of the release layer and also iscapable of receiving a laminating adhesive thereon to bond the monolayeror multilayer s release film to the other film, as described above.

In other applications the monolayer or multilayer, coextruded releasefilm may be used by itself, without being laminated to any other films.In this mode of operation, the surface of the film opposite the surfacethat is intended to function as the cold seal release surface preferablyis treated, preferably oxidatively treated by corona or flame treatment,to make this opposite surface receptive to cold seal adhesives and,optionally, to printing inks.

In accordance with the most preferred embodiments of this invention, thecold seal release film is an outer layer of a composite, multilayer,coextruded film including a central core, an outer release layerincluding a nucleating agent combined with the earlier-described polymercompositions and a skin layer on the side opposite the release layer. Asexplained above, the outer surface of this latter outer skin layer istreated, preferably oxidatively treated by corona or flame treatment, tomake the surface receptive to printing inks and/or laminating adhesivesand/or cold seal adhesives, depending upon whether this outer surface isintended to be reverse printed, laminated to an additional film thatwill include the cold seal adhesive on the outer exposed surfacethereof, or intended to directly receive the cold seal adhesive.

Preferably the skin layer opposite the release layer in the multilayer,coextruded release film is a propylene/ethylene copolymer including lessthan 10% ethylene, by weight, and more preferably less than 7% ethyleneby weight. A preferred propylene/ethylene copolymer employed as the skinlayer is Fina 8573, which, as noted earlier in this application,includes approximately 5% by weight ethylene monomer in it.

In a preferred embodiment of this invention the core layer is apolyolefin material, or a blend of polyolefin materials, and mostpreferably is a polypropylene homopolymer. A representativepolypropylene homopolymer employed as the core layer is Fina 3371.

A preferred multilayer construction of this invention is a three-layerstructure including a release film or layer having a thickness in therange of 0.25 to 20 microns; preferably 1 micron, a 20-micron-thick corelayer and 1-micron-thick skin layer.

It should be understood that the films of this invention may include avariety of additives that are well known to those skilled in the art.For example, antistatic additives, slip additives (e.g., high molecularweight fatty acid amides), organic or inorganic antiblocking additives(e.g., organic polymers such as polyamides, polyesters, polycarbonatesand inorganic materials such as silicon dioxide and silicates) andantioxidants may be added to one or more layers of the films of thisinvention, if desired.

Table 1, infra more specifically describes the construction andcomposition of seven sample multilayer, coextruded constructions;samples 2, 5 and 6 being within the scope of the present invention. Theremaining samples were prepared for comparison purposes.

Table 2, infra shows the cold seal release properties and the scuffresistence properties of the seven samples identified in Table 1. Itshould be noted that in order to achieve the maximum benefits of thisinvention, care should be taken to introduce the nucleating agentuniformly throughout the composition. This can best be achieved bygrinding the sodium benzoate or other nucleating agent to a very fineparticle size, on the order of less than 10 microns. Most preferably thenucleating agent employed in this invention is sodium benzoate having99.5%, by weight, of a particle size of 10 microns or less. TABLE 1Sample Descriptions Core Skin Release skin composition MaterialDescription Composition Material Description composition −1 Fina 3371homopolymer polypropylene Fina 3371 homopolymer polypropylene Fina 8573−2 Basell DP8220 medium ethylene content (2.5%) poly-1-butene Fina 3371homopolymer polypropylene Fina 8573 0.05% sodium benzoate −3 BasellDP8220 medium ethylene content (2.5%) poly-1-butene Fina 3371homopolymer polypropylene Fina 8573 0.0% sodium benzoate −4 Fina 3571homopolymer polypropylene Fina 3371 homopolymer polypropylene Fina 85730.3% Tospearl T-120 crosslinked polydialkyl siloxane 0.05% behenamide(antiblock agent) fatty acid amide (slip agent) −5 40% Basell DP8220medium ethylene content poly-1-butene Fina 3371 homopolymerpolypropylene Fina 8573 60% Fina 8573 ethylene propylene randomcopolymer 0.05% behenamide 0.1% sodium benzoate fatty acid amide 0.3%Tospearl T-120 crosslinked polydialkyl siloxane −6 40% Basell PB8340 lowethylene (0.2%) poly-1-butene Fina 3371 homopolymer polypropylene Fina8573 60% Union Carbide DS4DO butene propylene random copolymer 0.05%behenamide 0.1% sodium benzoate fatty acid amide 0.3% Tospearl T-120crosslinked polydialkyl siloxane −7 40% Basell DP8220 medium ethylenecontent poly-1-butene Fina 3371 homopolymer polypropylene Fina 8573 60%Fina 8573 ethylene propylene random copolymer 0.05% behenamide 0.0%sodium benzoate fatty acid amide 0.3% Tospearl T-120 crosslinkedpolydialkyl siloxanenote:all samples prepared with 1 micron release skin/20micron core/1 micronprint skin

TABLE 2 Cold Seal Adhesive Release Force g/in Sutherland 16 hr @ 22 C.,100 psi Rub test Release Skin Polymer ATOfindley ATOfindley TechnicalCoatings Technical Coatings Initial Final or Polymer Blend 1380 7114339D 357D % Haze % Haze −1 Homopolymer PP 18 30 90 144 1.4  1.5* −2Basell DP8220 + Sodium 10 20 35  54 5  6.3* Benzoate −3 Basell DP8220 +NO 17 nm nm nm 4.5 22.7* Sodium Benzoate −4 Homopolymer PP 41 35 73 1211.5  1.5** −5 40% DP8220 + 60% 42 30 35  64 1.2  1.2** Fina8573 + SodiumBenzoate −6 40% PB8340 + 60% 38 24 36  63 1  1.2** DS4D05 + SodiumBenzoate −7 40% DP8220 + 60% 35 28 38  60 1.2  2.5** Fina8573 + NOSodium BenzoateNotes:Samples 1-3 prepared on laboratory TMLong StretcherSamples 4-7 prepared on pilot tenter linenm—not measured*Sutherland Rub test or 50 cycles**Sutherland Rub test for 10 cycles

Film Testing Procedure to Determine the Cold Seal Adhesive Release Force

A coating of the specified adhesive was applied via a Mayer rod onto thedesignated film substrate to provide an adhesive strength greater than300 g/in. The coating was dried and then the release film was placedagainst the adhesive. Next, the two films were placed into a blockingjig where a contact force of 100 psi was applied. The samples were heldin the jig for 16 hours at 22° C. The force to separate the release filmfrom the adhesive was then measured.

Film Testing Procedure to Determine Scuff Resistance

Two sheets of film with the release surfaces facing each other weremounted on the base and sled of a Sutherland rub tester. The twosurfaces were then rubbed for the number of cycles specified in Table 2.The percent of transmission haze was measured before and after theSutherland rub test, and the difference is the measure of scuffresistance, i.e., the lower the difference the better the scuffresistance.

The invention is illustrated in more detail with reference to thefollowing Examples, but it should be understood that the presentinvention is not deemed to be limited thereto.

EXAMPLES

The test results reported in Table 2 demonstrate that the addition of anucleating agent, e.g., sodium benzoate, improves the scuff resistanceof the films of this invention, without adversely affecting cold sealrelease from both acrylic/rubber latex adhesives and syntheticadhesives.

Samples 1 through 3 were produced via a laboratory co-extrusion die andthen drawn 7× by 7× at a temperature of 140° C. to effect biaxialorientation. After orientation, the films were heat set 10%.Simultaneously biaxial orientation was carried out on a T.M. Longstretcher (T.M. Long Corporation, Somerville, N.J.).

Samples 4 through 7 were produced by a co-extrusion process on a pilottenter machine, with the cross-direction orientation step taking placeafter machine direction orientation, and with the machinedirection/cross-machine direction draw ratios being 5× by 10×.

As noted above, samples 1-3 were prepared on the laboratory extruder andT.M. Long stretcher. Samples 1 and 2 were evaluated with oneacrylic/rubber latex (ATOfindley 1380) and three synthetic adhesives(ATOfindley 7114, Technical Coatings 339D and Technical Coatings 357D).The comparative sample 3 was evaluated only with the acrylic/rubberlatex.

The ATO findley adhesives are manufactured by ATO Findley, WauwatosaWis. 53226. The ATOfindley 1380 adhesive is an acrylic/rubber latexadhesive. The ATOfindley 7114 adhesive is a synthetic adhesive.

The Technical Coatings adhesives are manufactured by Rohm & Haas,Woodstock Ill. 60098. Both the Technical Coatings 339D adhesive and theTechnical Coatings 357D adhesive are synthetic adhesives.

The test results of sample 2 versus sample I demonstrate that the1-butene/ethylene copolymer surface has release properties superior tohomopolymer polypropylene for synthetic adhesives, but has an inferiorscuff resistance. However, a comparison of sample 2 to comparativesample 3 demonstrates that the scuff resistance of the nucleated polymer(sample 2) is substantially superior to the non-nucleated polymer(comparative sample 3).

Samples 4 through 7 were prepared in a separate set of experiments on apilot tenter machine, as described earlier herein. In this sample set apolypropylene control, designated sample 4 was compared to nucleatedblends of 1-butene/ethylene copolymers with a random copolymer ofethylene and propylene designated sample 5, and with a random copolymerof butene and propylene designated sample 6. Comparative sample 7 is thesame polymer blend as sample 5 but with no sodium benzoate nucleatoradded. Note that due to differences in the manner of producing samples 1and 4 the absolute values of the adhesive release force are different.However, they still are of the same order of magnitude; demonstratingpoor release from synthetic adhesives.

A comparison of sample 4 with samples 5 and 6 again demonstrates thatblends containing 1-butene/ethylene copolymers yield superior cold sealrelease force for synthetic adhesives. Comparison of sample 5 withcomparative sample 7 again demonstrates that the addition of sodiumbenzoate nucleator improves the scuff resistance of the film (note thatsamples 1 through 3 were run for 50 cycles on the Sutherland rub testerand samples 4 through 7 were run for only 10 cycles).

While the invention has been described in detail with reference tospecific examples thereof, it will be apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

1. A packaging structure including a cold seal release film in the formof a monolayer film or an outer layer of a multilayer film, said coldseal release film having an outer surface and comprising a nucleatingagent and 10%-100% by weight of the polymer composition of a polymerselected from the group consisting of 1-butene homopolymer,1-butene/ethylene random copolymers having over 50% by weight 1-butenetherein and blends of the 1-butene homopolymer and the 1-butene/ethylenerandom copolymers, and 90%-0% by weight of the polymer composition of apolymer selected from the group consisting of random copolymers ofethylene and propylene, random copolymers of butene and propylene andrandom terpolymers of ethylene-butene-propylene, said propylene beingover 50% by weight of the polymer composition in said random copolymersof ethylene and propylene and random copolymers of butene and propyleneand being the major component by weight of the random terpolymers ofethylene-butene-propylene, and a cold seal adhesive being on an outersurface of the packaging structure opposed to the outer surface of saidcold seal release film.
 2. The packaging structure of claim 1, wherein100% of said polymer composition of said cold seal release film isselected from the group consisting of 1-butene homopolymer,1-butene/ethylene random copolymers having over 50% by weight 1-butenetherein and blends of 1-butene homopolymer and the 1-butene/ethylenerandom copolymers.
 3. The packaging structure, of claim 1, wherein 100%of said polymer composition of said cold seal release film is a1-butene/ethylene random copolymer having over 50% by weight 1-butenetherein.
 4. The packaging structure of claim 1 wherein the cold sealrelease film is the outer layer of a multilayer film.
 5. The packagingstructure of claim 2 wherein the cold seal release film is the outerlayer of a multilayer film.
 6. The packaging structure of claim 3wherein the cold seal release film is the outer layer of a multilayerfilm.
 7. The packaging structure of claim 1, wherein the outer surfaceof said cold seal release film opposed to the outer surface includingthe cold seal adhesive is a release surface.